Probability of precipitation
{{Short description|Estimated likelihood of precipitation occurring within a specified forecast region}}
{{Redirect|Chance of rain||Chance of Rain (disambiguation){{!}}Chance of Rain}}
Probability of precipitation (PoP) is a commonly used term referring to the likelihood of precipitation falling in a particular area over a defined period of time, which is commonly a day, half day, or hour.
The PoP measure is meaningless unless it is associated with an interval of time. Forecasts commonly use PoP defined over 12-hour periods (PoP12), though 6-hour periods (PoP6) and other measures are also published. A "daytime" PoP12 means from 6 am to 6 pm.{{cite web |last=Enyedi |first=Angie |url=http://www.srh.noaa.gov/jax/?n=probability_of_precipitation |archive-url=https://web.archive.org/web/20150610085838/http://www.srh.noaa.gov/jax/?n=probability_of_precipitation |archive-date=10 June 2015 |title=What are PoPs? |publisher=National Weather Service, Jacksonville, FL |date=27 August 2010 |accessdate=27 November 2018 }}
Probabilities are often calculated by ensemble forecasting and represents the number of simulations that show rain occurred.{{cite web | url=https://www.rmets.org/metmatters/what-does-30-chance-rain-mean | title=What does a 30% chance of rain mean? | date=23 May 2024 }}
PoPs are generally not statistically independent. A good example of an event that has a strongly dependent hour-to-hour PoP is a hurricane. In that case, there may be a 1 in 5 chance of the hurricane hitting a given stretch of coast, but if it does arrive there will be rain for several hours, with the effect that a one-hour PoP for the same region and period would be similar: about 1 in 5. Localized thunderstorms may be less dependent, with the effect that the one-hour PoPs may be somewhat less than the one-day PoP.{{cite news |last1=Gorski |first1=Chris |title=How Do Daily Weather Forecasts Relate To Hourly Forecasts? It Depends. |url=https://www.insidescience.org/news/how-do-daily-weather-forecasts-relate-hourly-forecasts-it-depends |accessdate=4 December 2018 |work=Inside Science |date=21 August 2014 |language=en}}
Definitions
= U.S. National Weather Service =
According to the U.S. National Weather Service (NWS), PoP is the probability of exceedance that more than {{convert|0.01|in|mm}} of precipitation will fall in a single spot, averaged over the forecast area.
The NWS provides hourly forecasts.{{cite web |last1=National Weather Service |title=Get your hourly weather forecast from the NWS |url=https://www.weather.gov/wrn/hourly-weather-graph |website=Weather.gov |accessdate=4 December 2018 |language=EN-US}} These forecasts include PoP over a given twelve-hour period (i.e. daytime vs night) which indicate a PoP if the given conditions at the start of the twelve-hour period hold for the next twelve-hours. They also include hour-to-hour forecasts, which is the PoP for that given hour. Because weather events hour to hour may be varyingly independent, hour-to-hour can be very similar to the current or twelve-hour PoP, or it can vary dramatically.
= Other US forecasters =
AccuWeather's definition is based on the probability at the forecast area's official rain gauge. There is also a probability of precipitation for every location in the United States for every minute for the next two hours. This is also known as a minute-cast. The Weather Channel's definition may include precipitation amounts below 0.01 inch (0.254 mm) and includes the chance of precipitation 3 hours before or after the forecast period. This latter change was described as less objective and more consumer-centric.{{cite news |url=https://blogs.wsj.com/numbers/deciphering-a-20-chance-of-rain-470/ |title=Deciphering a 20% Chance of Rain |date=2008-12-09 |first=Carl |last=Bialik |newspaper=The Wall Street Journal}} The Weather Channel has an observed wet bias – the probability of precipitation is exaggerated in some cases.{{cite news |url=https://www.nytimes.com/2012/09/09/magazine/the-weatherman-is-not-a-moron.html?pagewanted=all&_r=0 |title = The Weatherman Is Not a Moron |last = Silver |first = Nate |authorlink = Nate Silver |date = 2012-09-07 |newspaper = The New York Times}}
= Environment Canada =
Environment Canada reports a chance of precipitation (COP) that is defined as "The chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period."{{cite web |url=https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=B8CD636F-1&def=show02B55773D |title=Weather and Meteorology - Glossary: Chance of Precipitation (COP) |publisher=Environment Canada |accessdate=2015-06-07}} The values are rounded to 10% increments, but are never rounded to 50%.{{cite web |url=http://www.ec.gc.ca/meteo-weather/default.asp?lang=En&n=2D8EBDE4-1#c2 |title=Guide to Environment Canada's Public Forecasts: Chance of Precipitation |publisher=Environment Canada |accessdate=2015-06-07}}
= UK Met Office =
The UK's Met Office reports a POP that is rounded to 5% and is based on a minimum threshold of 0.1 mm of precipitation.{{cite web |url=https://www.metoffice.gov.uk/news/in-depth/science-behind-probability-of-precipitation |archive-url=https://web.archive.org/web/20181016041811/https://www.metoffice.gov.uk/news/in-depth/science-behind-probability-of-precipitation |archive-date=2018-10-16 |title=The science of 'probability of precipitation' |publisher=Met Office |date=2014-08-14}}
Alternative expressions
The probability of precipitation can also be expressed using descriptive terms instead of numerical values. For instance, the NWS might describe a precipitation forecast with terms such as "slight chance" meaning 20% certainty and "scattered" meaning 30–50% areal coverage.{{cite web |url=http://www.weather.gov/bgm/forecast_terms |title=Forecast Terms |publisher=National Weather Service, Binghamton, NY Weather Forecast Office |accessdate=2015-06-07}} The precise meaning of these terms varies.The NWS alone has published other sets of terms, such as those in [http://pajk.arh.noaa.gov/Articles/articles/survey/poptext.html NOAA Technical Memorandum NWS AR-44] and [http://www.nws.noaa.gov/directives/010/archive/pd01005003b.pdf NWS Instruction 10-503 (p. 18)].
The UK's Met Office replaced descriptive terms, such as "likely", with percentage chance of precipitation in November 2011.{{cite news |url=https://www.telegraph.co.uk/news/weather/8880438/Chance-of-a-shower-You-decide-as-Met-Office-launches-new-style-weather-forecasts.html |title=Chance of a shower? You decide, as Met Office launches new style weather forecasts |date=2011-11-10 |newspaper=The Telegraph}}
Public understanding
Probability of precipitation may be widely misunderstood by the general public.{{cite journal |title=Probability of Precipitation: Assessment and Enhancement of End-User Understanding |first1=Susan |last1=Joslyn |first2=Limor |last2=Nadav-Greenberg |first3=Rebecca M. |last3=Nichols |doi=10.1175/2008BAMS2509.1 |journal=Bulletin of the American Meteorological Society |volume=90 |issue=2 |pages=185–193 |date=February 2009|bibcode = 2009BAMS...90..185J |doi-access=free }}
The Plain English Campaign objected to the Met Office's use of the phrase "probability of precipitation" in 2011.{{cite web |url=http://www.plainenglish.co.uk/campaigning/awards/2011-awards/golden-bull-awards.html |title=2011 Golden Bull Awards |publisher=Plain English Campaign |accessdate=2015-06-07}}{{cite web |url=https://www.bbc.co.uk/news/uk-16100112 |title=Plain English award for Met Office 'gobbledygook' |publisher=BBC News |date=2011-12-09 |accessdate=2015-06-07}} The Met Office explained that the proposed alternative, "chance of rain", would not describe all the forms of precipitation included in the forecast.{{cite web |url=http://blog.metoffice.gov.uk/2011/12/09/a-golden-conundrum/ |title=A golden conundrum |publisher=Met Office |date=2011-12-09}}
See also
References
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Category:Numerical climate and weather models